Device and method for heating fuel cell stack and fuel cell system having the device

ABSTRACT

Provided is device and method for heating fuel cell stack and fuel cell system having the device. The fuel cell system includes: a power generating unit having fuel cell stacks arranged with an interval defined between the stacks; an outlet manifold unit provided outside each fuel cell stack and guiding a reaction mixture discharged from each stack to outside; an inlet manifold unit provided on each stack at a location opposed to the outlet manifold unit based on the stack, the inlet manifold unit supplying fuel and air supplied through a fuel supply pipe and an air supply pipe into the stack; and a subsidiary fuel supply unit for supplying subsidiary fuel into the outlet manifold unit such that the subsidiary fuel is burnt in the outlet manifold unit so as to heat both the outlet manifold unit and the stack coming into contact with the outlet manifold unit.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of Korean Patent Application No.10-2013-0121490, filed on Oct. 11, 2013, entitled “Device and Method forHeating the Fuel Cell and Apparatus having the Same”, which is herebyincorporated by reference in its entirety into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to device and method for heating a fuelcell stack and to a fuel cell system having the heating device.

2. Description of the Related Art

The deposits of fossil fuels, such as coal, gas and petroleum, generallyused as conventional energy sources are limited, so substitutionalenergy that can substitute for fossil fuels has become a great matter ofsocial and national concern and interest in recent years. For example,the need for power generation using solar heat, tidal power and windpower instead of fossil fuels, such as coal, gas and petroleum, or theneed for power generation using fuel cells is emphasized.

Of the proposed substitutional energy sources, fuel cells are designedto generate electricity using a reverse reaction of the electrolyticreaction of water. The fuel cells use a technology of converting oxygencontained in air and hydrogen contained in hydrocarbon-based materials,such as natural gas, coal gas and methanol, into electric energy throughan electrochemical reaction.

Unlike a conventional power generation technology requiring a variety ofprocesses, such as combustion of fuel, generation of steam, driving of aturbine, and driving of a power generator, the fuel cells neitherrequire combustion of fuel nor use driving devices, so the fuel cellsare advantageous in that the fuel cells can realize high operationalefficiency, produce few air pollutants, such as SOx and NOx, reduce theamount of carbon dioxide generated therefrom, and are less likely toproduce operational noises or vibrations.

Various kinds of fuel cells have been proposed and used in the relatedart. For example, phosphoric acid fuel cells (PAFC), alkaline fuel cells(AFC), polymer electrolyte membrane fuel cells (PEMFC), direct methanolfuel cells (DMFC), and solid oxide fuel cells (SOFC) have been proposedand used in the related art.

The solid oxide fuel cell (SOFC) is a fuel cell, in which a solid oxidethrough which oxygen ions or hydrogen ions can permeate is used as anelectrolyte. In the solid oxide fuel cell (SOFC), all the elementsconstituting the fuel cell are solid elements, so the solid oxide fuelcell is advantageous in that it has a simple construction, is free fromloss of the electrolyte, thereby requiring no replenishment of theelectrolyte, and is free from corrosion of other materials, compared toother type fuel cells. Further, the solid oxide fuel cell is operated ata high temperature, so the solid oxide fuel cell does not requireprecious metal catalysts, but fuel can be easily and efficientlysupplied to the fuel cell through a direct internal reforming process.Another advantage of the solid oxide fuel cell (SOFC) resides in thatthe fuel cell discharges high-temperature gas, so the solid oxide fuelcell can be efficiently used for combined heat and power generationusing waste heat.

In the solid oxide fuel cell (SOFC), electrode reactions expressed bythe following reaction formulas are performed.

<Reaction Formulas>Fuel electrode: H₂+O²⁻→H₂O+2e ⁻CO+O²⁻→CO₂+2e−Air electrode: O₂+4e−→2O²⁻Overall reaction: H₂+CO+O₂→H₂0+CO₂

In the fuel cell operated according to the above-mentioned reactionformulas, electrons reach the air electrode after passing through anexternal circuit, and, at the same time, oxygen ions generated from theair electrode move to the fuel electrode via the electrolyte, sohydrogen or CO is combined with the oxygen ions at the fuel electrode,thereby producing electrons and water or CO₂.

In a solid oxide fuel cell system, a stack that is formed by laminatinga plurality of unit cells is used as a base unit. To increase thecapacity of the solid oxide fuel cell system, a plurality of fuel cellstacks is connected to each other in series, in parallel or inseries-parallel.

FIG. 1 is a view illustrating the construction of a related art fuelcell system, in which problems of related art fuel cell stacks areshown.

As shown in FIG. 1, the related art fuel cell system 11 includes a powergenerating unit 19 formed by laminating a plurality of unit stacks 19 a,19 b and 19 c, a heat exchanger 13, a burner 15, a reformer 17, a powerrectifier 27, etc.

Here, the unit stacks 19 a, 19 b and 19 c are combined with each otherin a state in which laminated unit cells are enclosed in each unit stackthat comes into close contact with neighboring unit stacks. Electricpower generated by the power generating unit 19 is appropriatelyprocessed by the power rectifier 27, and is then supplied to an externaldevice requiring electric power.

The burner 15 receives gas from the back of the unit stacks 19 a, 19 band 19 c via a recycling pipe 25, and heats both the reformer 17 and theheat exchanger 13, and causes the reformer 17 to reform fuel, therebysupplying hydrogen-rich gas to the respective unit stacks of the powergenerating unit 19.

In the heat exchanger 13, heat is transferred from high-temperatureunreacted gas (hydrogen, air, etc.) that has been discharged from thepower generating unit 19 and recycled by the recycling pipe 25 to fueland air that have been newly introduced from the outside, therebyheating the fuel and air.

However, the related art fuel cell system 11 is problematic in that,because the unit stacks constituting the power generating unit 19 comeinto close contact with each other, deterioration in operationalperformance of a stack may easily ill-affect the other stacks placednear the deteriorated stack. In other words, a reduction in theperformance of a stack may be easily propagated to the other stacks.

Described in detail, when a problem occurs in a unit stack anddeteriorates the performance of the unit stack, a leaning of currentfrom the deteriorated stack to the other stacks placed near thedeteriorated stack is generated, so a heat balance between the unitstacks may be broken, thereby greatly reducing the performance of thefuel cell system 11. In other words, when the temperature of a unitstack is reduced (due to various abnormal reasons), temperatures ofneighboring unit stacks that come into close contact with thedeteriorated unit stack will be reduced, thereby inducing a greatreduction in the performance of the fuel cell system.

As well known to those skilled in the art, in a high-temperature fuelcell, such as SOFC, the operating temperatures of respective unit stacksimpose great effect on the operational performance of the stacks, suchas output power and durability of the stacks, so it is very important tomaintain desired operating temperatures of the unit stacks and tomaintain a heat balance between neighboring unit stacks. However, in theconventional power generating unit 19, the unit stacks are brought intoclose contact with each other, and no heating unit for increasing thetemperature of a unit stack to a normal temperature range when thetemperature of the unit stack is reduced is provided, so it is difficultto maintain optimal output power of the fuel cell system.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the related art, and the present inventionis intended to propose a fuel cell system, in which fuel cell stacks arearranged in a state of being spaced apart from each other, so heattransfer does not occur between the stacks, and the stacks are notthermally affected by each other, and in which inlet manifold units andoutlet manifold units are arranged between the stacks in such a way tothat the outlet manifold units functioning as heating sources canindividually heat the respective stacks, thereby protecting the stacksfrom problems that may be caused by a reduction in temperatures of thestacks, and maintaining optimal power generating efficiency of thestacks, and the present invention is also intended to propose a deviceand method for heating a fuel cell stack, in which the outlet manifoldunits can be individually heated.

In order to achieve the above object, according to one aspect of thepresent invention, there is provided a device for heating a fuel cellstack of a fuel cell system, in which, when the temperature of anoperating fuel cell stack has been reduced to a level out of a normaltemperature range, the device heats the stack so as to maintain thetemperature of the stack at a level within the normal temperature range,and which includes: an outlet manifold unit installed outside the fuelcell stack and guiding a reaction mixture discharged from the stack tooutside of the stack; and a subsidiary fuel supply unit for supplyingsubsidiary fuel into the outlet manifold unit such that the subsidiaryfuel is burnt in the outlet manifold unit so as to heat both the outletmanifold unit and the fuel cell stack coming into contact with theoutlet manifold unit.

The fuel cell stack may include two or more stacks that are arrangedwith an interval defined between the stacks, wherein an inlet manifoldunit may be provided on each of the stacks at a location opposed to theoutlet manifold unit based on the stack, the inlet manifold unitsupplying fuel and air into the stack

The outlet manifold unit may include therein: a cathode out part throughwhich a reaction mixture discharged from a cathode of the stack passes;and an anode out part separated from the cathode out part by a partitionwall, the anode out part receiving a reaction mixture discharged from ananode of the stack, so the reaction mixture passes through the anode outpart.

Further, the subsidiary fuel supply unit may include: a subsidiary fuelpipe for guiding the subsidiary fuel that is a part of fuel supplied tothe stack to an interior of the cathode out part of the outlet manifoldunit; and a flow control unit installed in the subsidiary fuel pipe andcontrolling a flow rate of the subsidiary fuel flowing through thesubsidiary fuel pipe.

The cathode out part of the outlet manifold unit may be provided with acombustion inducing unit for inducing combustion of the subsidiary fuelthat has been flowed into the cathode out part.

The device may further include: a control unit for controlling the flowcontrol unit; and a sensor provided on the stack so as to sense thetemperature of the stack, wherein the control unit controls the flowcontrol unit based on information about the temperature of the stackoutput from the sensor, thereby controlling the amount of suppliedsubsidiary fuel.

In the device, a combustion gas exhaust pipe may be mounted to thecathode out part and discharges combustion exhaust gas of the subsidiaryfuel generated from the cathode out part to the outside.

In another aspect, the present invention provides a method of operatinga device for individually heating fuel cell stacks, in which the deviceincludes: a power generating unit having a plurality of fuel cell stacksarranged with an interval defined between the stacks; an outlet manifoldunit provided outside each of the fuel cell stacks and guiding areaction mixture discharged from each of the stacks to outside of thestack; and a subsidiary fuel supply unit for supplying subsidiary fuelinto the outlet manifold unit such that the subsidiary fuel is burnt inthe outlet manifold unit so as to heat both the outlet manifold unit andthe stack coming into contact with the outlet manifold unit, and whichincludes: a primary temperature sensing operation for sensing atemperature of a fuel cell stack; a primary determining operation fordetermining whether the temperature of the stack sensed at the primarytemperature sensing operation is within a normal temperature range ornot; a manifold heating operation for supplying subsidiary fuel from asubsidiary fuel supply unit to an outlet manifold unit and heating theoutlet manifold unit when it is determined at the primary determiningoperation that the temperature of the stack is not within the normaltemperature range; a secondary temperature sensing operation for sensinga temperature of the stack heated at the manifold heating operation; asecondary determining operation for determining whether the temperatureof the stack sensed at the secondary temperature sensing operation iswithin the normal temperature range or not; and a subsidiary fuelblocking operation for blocking a flow of the subsidiary fuel bycontrolling the subsidiary fuel supply unit when it is determined at thesecondary determining operation that the sensed temperature of the stackis within the normal temperature range.

Further, the outlet manifold unit may include: a cathode out partthrough which a reaction mixture discharged from a cathode of the stackpasses; and an anode out part separated from the cathode out part by apartition wall, the anode out part receiving a reaction mixturedischarged from an anode of the stack, so the reaction mixture passesthrough the anode out part, and the subsidiary fuel supply unitincludes: a subsidiary fuel pipe for guiding the subsidiary fuel that isa part of fuel supplied to the stack to an interior of the cathode outpart of the outlet manifold unit; and a flow control unit installed inthe subsidiary fuel pipe and controlling a flow rate of the subsidiaryfuel flowing through the subsidiary fuel pipe. Here, the manifoldheating operation may be an operation for supplying the subsidiary fuelinto the cathode out part by controlling the flow control unit.

Further, the subsidiary fuel blocking operation may be an operation forblocking the flow of the subsidiary fuel flowing to the cathode out partby closing the flow control unit.

In a further aspect, the present invention provides a fuel cell system,including: a power generating unit having a plurality of fuel cellstacks arranged with an interval defined between the stacks; an outletmanifold unit provided outside each of the fuel cell stacks and guidinga reaction mixture discharged from each of the stacks to outside of thestack; an inlet manifold unit provided on each of the stacks at alocation opposed to the outlet manifold unit based on the stack, theinlet manifold unit supplying fuel and air which are supplied fromoutside through a fuel supply pipe and an air supply pipe into thestack; and a subsidiary fuel supply unit for supplying subsidiary fuelinto the outlet manifold unit such that the subsidiary fuel is burnt inthe outlet manifold unit so as to heat both the outlet manifold unit andthe stack coming into contact with the outlet manifold unit.

Here, the outlet manifold unit may include therein: a cathode out partthrough which a reaction mixture discharged from a cathode of each ofthe stacks passes; and an anode out part separated from the cathode outpart by a partition wall, the anode out part receiving a reactionmixture discharged from an anode of the stack, so the reaction mixturepasses through the anode out part.

Further, the subsidiary fuel supply unit may include: a subsidiary fuelpipe for guiding the subsidiary fuel that is a part of fuel supplied toeach of the stacks to the interior of the cathode out part of the outletmanifold unit; and a flow control unit installed in the subsidiary fuelpipe and controlling the flow rate of the subsidiary fuel flowingthrough the subsidiary fuel pipe.

Further, the cathode out part of the outlet manifold unit may beprovided with a combustion inducing unit for inducing combustion of thesubsidiary fuel that has been flowed into the cathode out part.

Further, the fuel cell system may further include: a control unit forcontrolling the flow control unit; and a sensor provided on each of thestacks so as to sense a temperature of each of the stacks, wherein thecontrol unit controls the flow control unit based on information aboutthe temperature of the stack output from the sensor, thereby controllingthe amount of supplied subsidiary fuel.

Further, a combustion gas exhaust pipe may be mounted to the cathode outpart and discharges combustion exhaust gas of the subsidiary fuelgenerated from the cathode out part to the outside.

The fuel cell system may further include: a heat exchanger providedbetween the combustion gas exhaust pipe and the air supply pipe so as totransfer heat of the combustion gas exhaust pipe to the air supply pipe.

Further, the fuel cell system may further include: a recycling pipemounted to the anode out part, the recycling pipe receiving and guidinga reaction mixture, thereby discharging the reaction mixture to theoutside after the reaction mixture passes through the anode out part;and a heat exchanger provided between the recycling pipe and the fuelsupply pipe.

Various objects, advantages and features of the invention will becomeapparent from the following description of embodiments with reference tothe accompanying drawings.

The terms and words used in the present specification and claims shouldnot be interpreted as being limited to typical meanings or dictionarydefinitions, but should be interpreted as having meanings and conceptsrelevant to the technical scope of the present invention based on therule according to which an inventor can appropriately define the conceptof the terms to describe most appropriately the best method he or sheknows for carrying out the invention.

In the above-mentioned fuel cell system according to the presentinvention, the fuel cell stacks are arranged in a state of being spacedapart from each other, so heat transfer does not occur between thestacks, and the stacks are not thermally affected by each other, and inwhich the inlet manifold units and the outlet manifold units arearranged between the stacks in such a way that the outlet manifold unitsfunctioning as heating sources can individually heat the respectivestacks, so the present invention can protect the stacks from problemsthat may be caused by a reduction in temperatures of the stacks, and canmaintain optimal power generating efficiency of the stacks.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description when taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a view illustrating the construction of a related art fuelcell system, in which problems of related art fuel cell stacks areshown;

FIG. 2 is a view illustrating the construction of a fuel cell systemaccording to an embodiment of the present invention, in which theoperational theory of an individual heating device configured toindividually heat fuel cell stacks is shown;

FIG. 3 is a view illustrating the operational theory of the individualheating device of the fuel cell system of FIG. 2 in more detail; and

FIG. 4 is a view illustrating a method of individually heating the fuelcell stacks according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings.

Reference now should be made to the drawings, in which the samereference numerals are used throughout the different drawings todesignate the same or similar components.

Further, it will be understood that, although the terms “first,”“second,” etc. may be used herein to describe various elements, theseelements should not be limited by these terms.

Further, when it is determined that the detailed description of theknown art related to the present invention might obscure the gist of thepresent invention, the detailed description thereof will be omitted.

Hereinbelow, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 2 is a view illustrating the construction of a fuel cell system 31according to an embodiment of the present invention, in which theoperational theory of an individual heating device configured toindividually heat the fuel cell stacks is shown. FIG. 3 is a viewillustrating the operational theory of the individual heating device ofthe fuel cell system 31 shown in FIG. 2, in more detail.

As shown in FIGS. 2 and 3, the fuel cell system 31 according to theembodiment of the present invention includes: a power generating unit 51having a plurality of fuel cell stacks 53 that are sequentially arrangedin a vertical direction with intervals defined between the stacks; inletmanifold units 55 closely installed on respective stacks 53; outletmanifold units 57 closely installed below the respective stacks 53; andan individual heating device configured such that, when the temperatureof an operating fuel cell stack is reduced, the individual heatingdevice individually heats the relevant stack, thereby maintaining thetemperature of the relevant stack within a normal temperature range.

The individual heating device includes a subsidiary fuel supply unitthat can supply subsidiary fuel into a selected outlet manifold unit 57,so the subsidiary fuel can be burnt in the outlet manifold unit 57 andcan heat both the outlet manifold unit and a fuel cell stack 53 thatcontacts with the outlet manifold unit.

The stacks 53 constituting the power generating unit 51 comprise an nnumber of stacks that are arranged on top of one another with aninterval defined between the stacks 53. Further, laminated unit cellsare enclosed in each of the stacks 53. In other words, the constructionof each of the stacks 53 remains the same as a conventional stack.

The fuel cell system 31 of this embodiment is characterized in that thestacks 53 are arranged in a state in which each of the stacks 53 isspaced apart from neighboring stacks 53, with an inlet manifold unit 55and an outlet manifold unit 57 interposed between two neighboring stacks53.

Because the stacks 53 are spaced from each other as described above, noheat transfer occurs between the stacks 53. Further, the outlet manifoldunits 57 and associated inlet manifold units 55 are combined with eachother, and mechanically fix the stacks 53 to each other. Particularly,each of the outlet manifold units 57 is heated by subsidiary fuelsupplied from an external source, and heats an associated stack 53placed thereon.

Here, each of the inlet manifold units 55 is connected both to an end ofa fuel supply pipe 33 and to an end of an air supply pipe 47. Thus, fuelthat has been supplied to the inlet manifold unit 55 via the fuel supplypipe 33 after passing a reformer 45 is supplied to a fuel electrode (notshown) inside each of the stacks 53 after passing the inlet manifoldunit 55, and air that has been supplied to the inlet manifold unit 55via the air supply pipe 47 is supplied to an air electrode inside eachof the stacks 53 after passing through the inlet manifold unit 55, sothe fuel and air can participate in an electrochemical reaction.

Further, the outlet manifold units 57 function to guide the reactionmixture (water, unreacted hydrogen, air, etc.) discharged from thestacks 53 to the atmosphere. Here, the reaction mixture passes throughthe outlet manifold units 57 prior to being guided to the atmosphere.

Particularly, as shown in FIG. 3, each of the outlet manifold units 57includes a cathode out part 57 a and an anode out part 57 which areseparated from each other by a partition wall 57 c. Here, the cathodeout part 57 a is a chamber through which the reaction mixture generatedfrom the cathode of an associated stack 53 passes, and the anode outpart 57 b is a chamber that receives the reaction mixture generated fromthe anode of an associated stack 53 and guides the reaction mixture to arecycling pipe 65.

Further, each of the cathode out parts 57 a of the outlet manifold units57 is provided with a combustion inducing unit 67 therein. Thecombustion inducing unit 67 functions to promote firing of thesubsidiary fuel that has flowed into the cathode out part 57 a, so theunit 67 induces combustion of the subsidiary fuel. Here, adiffusion-type combustor or a catalytic combustor may be used as thecombustion inducing unit 67.

During the operation of the fuel cell system 31, the outlet manifoldunits 57 have been heated to a high temperature not less than 600° C.due to the continuous operation of the power generating unit 51 (theinlet manifold units 55 heated in the same manner), so the subsidiaryfuel will be naturally and immediately ignited after the subsidiary fuelflows into the cathode out parts 57 a. Accordingly, the combustioninducing units 67 may be omitted from the fuel cell system 31.

The individual heating device is configured such that, when thetemperature of an operating fuel cell stack of the power generating unit51 is lowered, the individual heating device individually heats therelevant stack, thereby maintaining the temperature of the relevantstack within a normal temperature range.

The individual heating device having the above-mentioned functionincludes the subsidiary fuel supply unit that can supply the subsidiaryfuel to the cathode out part 57 a of a selected outlet manifold unit 57.

The subsidiary fuel supply unit includes: a subsidiary fuel pipe 35 thatguides subsidiary fuel which is a part of fuel that flows from the fuelsupply pipe 33 to the respective stacks 53 to the interior of a selectedcathode out part 57 a, and a flow control unit 37 that is installed inthe subsidiary fuel pipe 35 and controls the flow rate of the subsidiaryfuel flowing through the subsidiary fuel pipe 35. Here, the flow controlunit 37 includes a plurality of valves 37 a.

Here, the subsidiary fuel is supplied to the cathode out parts 57 in astate before being reformed by the reformer 45. When the subsidiary fuelflows into a high temperature cathode out part 57 a, the subsidiary fuelis naturally ignited and heats the outlet manifold unit 57. Here, theoutlet manifold unit 57 comes into close contact with the lower surfaceof an associated stack 53, so, when heating the outlet manifold unit 57as described above, the outlet manifold unit 57 can heat the stack 53.

The flow rates of the subsidiary fuel supplied to the cathode out parts57 a will be increased in proportion to the opening ratios of the valves37 a. When completely closing the valves 37 a, no subsidiary fuel willbe supplied to the cathode out parts 57 a.

The valves 37 a may be manual valves or solenoid valves that will beoperated in response to electric signals output from the control unit39.

Here, sensors 61 are installed on the side surfaces of the respectivestacks 53. The sensors 61 function to sense the temperatures of therespective stacks 53 in real time. Information about the temperaturessensed by the sensors 61 is supplied to an operator via a display unit(not shown).

Further, the control unit 39 determines whether it is required to open avalve 37 a or not based on information about temperatures output fromthe sensors 61. When the temperature of a stack 53 is lower thantemperatures of neighboring stacks, the control unit 39 opens a valve 37a associated with the low temperature stack 53, thereby supplyingsubsidiary fuel to the cathode out part 57 a of the outlet manifold unit57 that comes into contact with the low temperature stack 53.

Unreacted hydrogen contained in the respective anode out parts 57 b ofthe outlet manifold units 57 moves through the recycling pipe 65, and issupplied to the inlet manifold units 55 after passing through a heatexchanger 41 a, a blower 43 and the reformer 45. Here, the function ofthe blower 43 is to add kinetic energy to hydrogen gas, thereby feedingthe hydrogen gas to the reformer 45 under pressure.

Both the recycling pipe 65 and the fuel supply pipe 33 commonly passthrough the heat exchanger 41 a, so heat exchange between the recyclingpipe 65 and the fuel supply pipe 33 occurs in the heat exchanger 41 a.In other words, fuel that flows from an external source to the reformer45 through the fuel supply pipe 33 is heated by high-temperaturehydrogen gas that moves through the recycling pipe 65.

Further, combustion gas exhaust pipes 63 extend from the respectivecathode out parts 57 a. Here, the combustion gas exhaust pipes 63function to discharge combustion exhaust gas from the cathode out parts57 a to the atmosphere.

Another heat exchanger 41 b is provided between the combustion gasexhaust pipe 63 and the air supply pipe 47. The function of the heatexchanger 41 b is to perform heat exchange between high-temperaturecombustion exhaust gas flowing through the combustion gas exhaust pipe63 and air that is newly supplied through the air supply pipe 47.

FIG. 4 is a view illustrating a method of individually heating the fuelcell stacks according to an embodiment of the present invention. Themethod of individually heating the fuel cell stacks according to theembodiment of the present invention is to drive the individual heatingdevice of the fuel cell system.

As shown in FIG. 4, the method of individually heating the fuel cellstacks according to the embodiment of the present invention starts atprimary temperature sensing operation 100 in which the temperatures ofthe respective stacks 53 are sensed by the sensors 61. Information abouttemperatures of the stacks 53 sensed in real time at primary temperaturesensing operation 100 is given to an operator and is also applied to thecontrol unit 39.

After finishing primary temperature sensing operation 100, primarydetermining operation 102 is performed. Here, primary determiningoperation 102 is an operation for determining whether the presenttemperatures of the stacks 53 sensed at primary temperature sensingoperation 100 are within a normal temperature range or not. At primarydetermining operation 102, the determination may be performed by thecontrol unit 39 or by an operator. Particularly when general type manualvalves are used as the valves 37 a, the determination at primarydetermining operation 102 will be performed by an operator.

When it is determined at primary determining operation 102 that thetemperature of a stack is not within the normal temperature range,manifold heating operation 104 is performed in which a valve 37 aassociated with the relevant stack is opened, thereby supplyingsubsidiary fuel to an associated cathode out part 57 a and heating anassociated outlet manifold. Here, the valve 37 a may be manually openedby an operator or may be automatically opened under the control of thecontrol unit 39.

However, when is determined at primary determining operation 102 thatthe temperature of a stack is within the normal temperature range,primary temperature sensing operation 100 is repeated without proceedingto another operation.

After finishing manifold heating operation 104, Secondary temperaturesensing operation 108 is performed. Here, secondary temperature sensingoperation 108 is an operation for sensing the temperature of theindividually heated stack 53 using an associated sensor 61. Informationabout the temperature of the individually heated stack sensed atsecondary temperature sensing operation 108 is given to the operator andis also applied to the control unit 39. The process of secondarytemperature sensing operation 108 is equal to that of primarytemperature sensing operation 100.

Secondary determining operation 110 is performed after finishingsecondary temperature sensing operation 108. Here, secondary determiningoperation 110 is an operation for determining whether the temperature ofthe stack 53 individually heated at manifold heating operation 104 iswithin the normal temperature range or not.

When it is determined at secondary determining operation 110 that thetemperature of the stack 53 individually heated at manifold heatingoperation 104 is not within the normal temperature range, manifoldheating operation 104 is repeated.

However, when it is determined that the temperature of the individuallyheated stack 53 is within the normal temperature range, subsidiary fuelblocking operation 112 is performed in which supply of subsidiary fuelis stopped by controlling the subsidiary fuel supply unit. Here,subsidiary fuel blocking operation 112 is an operation for blocking theflow of subsidiary fuel that flows to the associated cathode out part 57a by closing the associated valve 37 a.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

Further, simple changes and modifications of the present invention areappreciated as included in the scope and spirit of the invention, andthe protection scope of the present invention will be defined by theaccompanying claims.

What is claimed is:
 1. A fuel cell system, comprising: a powergenerating unit having a plurality of fuel cell stacks arranged with aninterval defined between each of the stacks; an outlet manifold unitprovided below each fuel cell stack, each outlet manifold unitincluding: a cathode out part through which a first reaction mixturedischarged from a cathode of the associated stack passes; and an anodeout part through which a second reaction mixture discharged from ananode of the associated stack passes, wherein for each outlet manifoldunit, the anode out is separated from the cathode out part by apartition wall; an inlet manifold unit provided on each of the stacks ata location opposed to the outlet manifold unit based on the stack, theinlet manifold unit supplying fuel and air which are supplied fromoutside through a fuel supply pipe and an air supply pipe into eachstack; and a subsidiary fuel supply unit for supplying subsidiary fuelinto each cathode out manifold unit such that the subsidiary fuel isburnt in each cathode out manifold unit so as to heat each outletmanifold unit and its associated stack.
 2. The fuel cell system as setforth in claim 1, wherein the subsidiary fuel supply unit includes: asubsidiary fuel pipe for each stack for guiding the subsidiary fuel thatis a part of the fuel supplied to each stack to an interior of thecathode out part associated with the outlet manifold unit of that stack;and a flow control unit installed in each subsidiary fuel pipe andcontrolling a flow rate of the subsidiary fuel flowing through theassociated subsidiary fuel pipe.
 3. The fuel cell system as set forth inclaim 2, wherein the cathode out part of each outlet manifold unit isprovided with a combustion inducing unit for inducing combustion of thesubsidiary fuel that has been flowed into the associated cathode outpart.
 4. The fuel cell system as set forth in claim 2, furthercomprising: a control unit for controlling the plurality of flow controlunits; and a sensor provided on each of the stacks so as to sense atemperature of each of the stacks, wherein the control unit controlseach flow control unit based on information about the temperature of thestack output from the related sensor, thereby controlling an amount ofsupplied subsidiary fuel.
 5. The fuel cell system as set forth in claim2, wherein a combustion gas exhaust pipe is mounted to each cathode outpart and discharges combustion exhaust gas of the subsidiary fuelgenerated from the cathode out part.
 6. The fuel cell system as setforth in claim 5, further comprising: a heat exchanger provided betweenthe plurality of combustion gas exhaust pipes and the plurality of airsupply pipes so as to transfer heat of the plurality of combustion gasexhaust pipes to the plurality of air supply pipes.
 7. The fuel cellsystem as set forth in claim 6, further comprising: a recycling pipeconnected to each anode out part, the recycling pipe receiving anddischarging the second reaction mixture from each anode out part; and aheat exchanger provided between the recycling pipe and the fuel supplypipe.